1. Field of the Invention
The present invention relates to an active object detection sensor that transmits detection rays for object detection toward a detection area, and detects an object based on a reception signal generated when receiving detection rays reflected by the object.
2. Description of Related Art
To date, an active object detection sensor has been known which transmits detection rays, such as infrared rays or near infrared rays, for object detection, from a transmitter toward a detection area, causes a receiver to receive detection rays reflected by an object and to generate a reception signal, and detects an object such as a human body when the reception signal is determined as having a level that exceeds a setting level.
As an example of the active object detection sensor, the use is made of a sensor that forms a detection area divided into a plurality of division areas, to arrange plural columns of the division areas in the lateral direction, and arrange plural rows of the division areas in the longitudinal direction from a position close to the sensor toward a position distant from the sensor (for example, Japanese Laid-Open Patent Publication No. 2009-115792). The active object detection sensor is, for example, used for an automatic door sensor so as to detect an object for the automatic door.
FIG. 3A shows an example of a sensor. The sensor includes three phototransmitter (transmitter) elements 3 (elements EA to EC) and three photodetector (receiver) elements 4 (elements Ea to Ec) that are paired, respectively, so as to form three pairs. On the phototransmitter (transmission) side, a light transmission area (transmission area) TA having six division areas is formed by the three phototransmitter elements 3, a lens body 5, and a phototransmitter-side prism 7. On the photodetector (reception) side, a light reception area (reception area) RA having six division areas is formed by the three photodetector elements 4, a lens body 6, and a photodetector-side prism 8. The light transmission area TA and the light reception area RA are overlaid or overlapped on each other to form a detection area AA. The prisms 7 and 8 each have, for example, a plurality of triangular portions each having two prism surfaces.
For example, a detection ray from an element EA of the phototransmitter element 3 is diffused by the two prism surfaces of the prism 7 in two directions, to form an area A of a division area TA11 and an area A′ of a division area TA14 of the light transmission area TA, and detection rays are received by an element Ea of the photodetector element 4 via the two prism surfaces of the prism 8 from two directions, that is, from an area a of a division area RA11 and an area a′ of a division area RA14 of the light reception area RA. A division area A1 of a first row or line is formed by the light transmission area TA11 and the light reception area RA11, and a division area A2 of a second row or line is formed by the light transmission area TA12 and the light reception area RA12. As shown in FIG. 3B, division areas A1 to A6 in which the light transmission areas TA11 to TA16 and the light reception areas RA11 to RA16 of six lines are overlaid or overlapped on each other in the longitudinal direction, are aligned from a position close to a sensor 100 toward a position distant from the sensor 110, to form the detection area AA. It is to be noted that a plurality of division areas are formed in the lateral direction for each of the six rows or lines of the division areas A1 to A6, which lateral division areas are not shown.
FIG. 6A illustrates an example of a conventional sensor. In general, areas of two prism surfaces of a prism P are set such that a ratio between an area of a prism surface Pb by which a ray is applied to a rear portion corresponding to a short distance portion (closer to the sensor), and an area of a prism surface Pa by which a ray is applied to a front portion corresponding to a long distance portion (farther from the sensor) is 50:50 on each of the phototransmitter side and the photodetector side respectively. Therefore, as shown in FIG. 6B and FIG. 6C, the division areas RA11 and RA16, on the photodetector side, which are positioned at a position close to the sensor 110 having the prism P and a position distant from the sensor 110 having the prism P, respectively, are formed by prism surfaces having the same area, and the division areas TA11 and TA16, on the phototransmitter side, which are positioned at a position close to the sensor 110 having the prism P and a position distant from the sensor 110 having the prism P, respectively, are formed by prism surfaces having the same area.
When this sensor is used as an automatic door sensor, the longer a distance between the sensor and a division area of the detection area is, that is, the farther the division area of the detection area is from the automatic door toward the external region, the lower the detection sensitivity is likely to become. Therefore, in a case where a door is to be immediately opened for a person approaching the automatic door from the external region, a detection sensitivity for the distant division area is low and detection becomes difficult, to reduce a response speed. In this case, it can be considered that a detection sensitivity is enhanced. However, enhancement of detection sensitivity leads to increase of an influence of a noise, and an S/N ratio cannot be improved, to make improvement of detection performance difficult.
On the one hand, in a division area close to the automatic door, detection sensitivity is unnecessarily enhanced due to a short distance from the sensor, so that malfunction is likely to occur. Thus, in the case as described above, in the distant division area, detection performance cannot be improved, whereas, in the close division area, detection sensitivity is unnecessarily enhanced to increase the number of times of malfunction, and the sensor is impractical.
On the other hand, it is known that, in order to make the detection sensitivity uniform over the entirety of the detection area, an angle of the prism surface is varied (for example, Japanese Laid-Open Patent Publication No. 2004-170128). However, a mechanism for moving or rotating a prism the angle of which is gradually varied is necessary, and detection performance cannot be sufficiently improved for the distant division area.